This invention relates generally to contact lenses, and particularly to an improved lens and method and apparatus for making same.
The design, development and production of contact lenses are representative activities of the class of so-called "high technology" industries. The research competition for improved materials and techniques is very keen and large sums of monies are expended each year by companies to insure that their abilities are better than, or commensurate with, others in the market place. The instant invention is concerned not with materials, but rather the production of improved lenses.
The ideal end product in this art is one which is not only comfortable to wear for extended periods of time, but also easily manufactured at minimum cost in time and labor. Importantly, any specific lens fitting characteristics should also be readily reproducable in a consistent manner to satisfy the requirements of the replacement buyer.
Early corneal contact lenses, i.e., lenses designed and fitted to cover only the corneal region of the eye, while being revolutionary in concept, nonetheless displayed a number of disadvantages which were later overcome as technology improved. The cornea of the eye is not truly spherical edge to edge, but is so only in its central portion. From the edge of the central position outwardly toward the limbal area, the curvature of the cornea flattens or its radius increases. If a lens having a curvature parallel with the central portion of the cornea is used, the edges of the lens will rest on the peripheral cornea, shutting off circulation and free flow of lachrymal fluids and also hindering the cornea from obtaining oxygen from the atmosphere, both of which are necessary for lens tolerance. In the absence of the free circulation of oxygen and oxygen enriched fluids between the lens and the eye, there is a veiling and fogging of vision, and the wear time is greatly limited. If a lens with a base curve or inside curvature flatter than the curve of the actual portion of the cornea and of sufficient length to provide the required inside contour to clear the limbal cornea area is used, such a lens will rest on the apex of the cornea thus exerting pressure which will temporarily flatten the cornea. Aside from producing a temporary blurred vision, this latter arrangement allows little capillary attraction and may not remain in position possibly resulting in corneal abrasions and ulcerations.
To overcome the above-described problems, the most successful modern contact lenses are produced with the base curve slightly steeper than the central corneal curves and with the posterior surface near the edge flatter than the base curve. This special edge treatment is intended to creat an ideally positioned reservoir for oxygen enriched tears to collect, ready to flow under the lens when it is set in motion by a blink of the eye. This area was referred to in early literature as a "chamfer", but is now referred to as a "bevel".
It is well accepted that the ideal bevel configuration resembles a ski tip; hence, reference is often made to the "ski" bevel. Prior to the instant invention, the "ski" bevel (see FIG. 1) was merely the theoretical ideal toward which all manufacturers directed their efforts.
Present conventional bevels are produced by grinding the posterior surface of an unfinished lens at its periphery with revolving, spherical tools which have surface curvatures flatter than the base curve of the lens. Actually, a series of tools, each tool having a progressively flatter curve, is used in an attempt to "blend" the work together into a surface as smooth as possible. The process is generally viewed as a simple one. It is accomplished by using uncomplicated equipment consisting of a motor driven spindle, a polishing pot to contain the spray of polish, and a few interchangeable beveling tools. With this equipment which occupies less than two square feet of table space, one can bevel and finish contact lenses. It is important to note that the same basic equipment and techniques are used by all contact lens manufacturers.
Production of the ideal bevel has been a major problem for the contact lens industry since the beginning. The problem has been how to best produce a uniformly shaped beveled surface with spherical beveling tools. The prior art system normally does not result in controllable, smoothly beveled surfaces. Instead the majority of lenses that are produced have had irregularly beveled surfaces, representing infinite variations of wavy surface configurations. This has been proven through the random evaluation of the lenses in the field with a profile analyzer or by using the reflection from an illuminated fluorescent light source. In fact, previous literature on the subject has encouraged fitters to inspect the lenses they receive for bevel irregularities.
Referring to FIG. 3a, it can be seen that grinding a bevel with a single, spherical tool creates a sharp junction 32 dividing the lens base curve 30 and the bevel 34. Early clinical experience proved this to be a serious lens defect. In order to correct this condition, mid-range tool must be selected and employed to grind down the sharp juncture. This is called blending. However, this step results in the creation of two new junctures of lesser magnitude which now must be ground down, so, the process is repeated with other tools in an attempt to obtain a uniformly curved surface. See, for example, FIGS. 3b, through 3d, where the process of grinding successive curves 36, 42 and 44 results in the creation of junctures 46, 48, 50 and 52. Beveling, then, has been basically a step-by-step procedure of trying to make the interfacial curve and junctions as small and uniform as possible.
Theoretically, the step-by-step beveling process of using a series of progressively flatter tools seems to be practical enough to do an adequate job, and it probably would be, if it were not for other elements that caused serious problems and poor end results.
First, selecting the proper number of tools and radii for each lens base curve is one thing, but determining the exact grinding time with each tool is another. Secondly, the precise grinding pressure cannot be determined even by the most skilled technician. In manufacturing, it is these uncontrollable variables together with the other things mentioned that result in a beveling system which produces entirely unsuitable lens characteristics, such as exemplified at 22 in FIG. 2.
In summary, the conventional beveling process exhibits many serious drawbacks, including: (a) the beveled surfaces cannot be adequately controlled, (b) the majority of lenses manufactured have serious beveled defects, especially sharp junctions and wavy interfaces, (c) the topography of the beveled surface cannot be duplicated in the event of a lost lens, and (d) the slope of the bevel which determines the volume of the tear reservoir cannot be adequately controlled.